I suggest a lot more specific. A bookmark can have paragraphs, images, tables, textboxes, etc. all in it. It can also start in the middle of a table and end outside the table. So replacing what's inside it can be very problematic.
So I'm going to take a guess as to what you want and from that might have an answer for you. I am guessing you want something where you place tags in the document and then your program can replace those tags with data. Instead of bookmarks use fields. There are a number of mailmerge fields that work great for this.
If this will work for you, then for the actual code, Descendents is the main thing you need.